Accelerators without batteries

Accelerators use cavities (the structures running through the center of the tunnel) to kick particle beams to higher and higher energies. Photo: Reidar Hahn

In my last column I discussed scattering experiments as a major tool for gathering information about our world. I suggested that we could get more detailed information than our light beams, in combination with our eyes, can provide if we devised more powerful beams and detectors. We concluded that providing beam energy by using flashlight batteries is not only impractical, but is innovative only in a literary sense. Powerful beams require technical innovation.

The upgrade of our beams from visible light to higher-energy particles began about 100 years ago when two scientists named Cockcroft and Walton built an electrostatic generator capable of producing 700,000 volts. By this time we were already using electron beams and beams from natural radioactive sources to investigate nature. For example, in 1909 Ernest Rutherford discovered the nucleus of the atom using a beam of alpha particles from a radioactive source.

Cockcroft and Walton used their impressive voltage generator to investigate the details of the atomic nucleus, discovering the neutron in the process. One disadvantage of this brute-force technique is that it works only for voltages less than a million volts, corresponding to 1 million electronvolts of energy to the beam, limiting the detail that could be studied.

Early particle physicists overcame the voltage constraints by using two techniques still used in modern accelerators. The first technique resembles the battery solution that proved so impractical in my last column. This time I will be more clever: The second technique makes use of a series of accelerating stations, made up of resonant cavities arranged end to end and tuned to oscillate at a particular frequency.

An oscillating voltage rapidly cycles from positive to negative. Coupled to each cavity, it provides an acceleration kick to a bunch of beam particles timed to arrive at the right point in the cycle. The resonant cavities maximize the acceleration available from the power supplied from the wall plug. In this manner we can achieve peak accelerating voltages of more than a million volts depending on the size and shape of the cavity and the frequency of the oscillations. Particles arriving nearer to the voltage peak receive a larger acceleration. By arranging the phase of the voltage oscillations with respect to the beam particles, we can give a larger kick to the late arriving particles and a smaller kick to the early arrivals. (Sometimes Einstein and his relativity theory demand that we do just the opposite. I will explain this in a subsequent column.)

We now have the primary innovation that we need to construct a linear accelerator, in which beam runs in a straight line from one end of the cavity string to the other exactly once. However, linear accelerators have their limitations as well as advantages. Next time we will move from straight-line to circular acceleration. By now I hope it is obvious that running the beam through the same accelerating cavities over and over might be a good idea. If not, stay tuned!

Roger Dixon